Railway well cars may be conceptualised as having a pair of deep, spaced apart, parallel beams, with floor members extending cross-wise between the beams to form a support frame for lading. The ends of the deep beams are mounted to end structures, and the end structures are supported on a pair of railcar trucks. Although single unit well cars are still common, there has been a trend in recent years toward articulated, multi-unit railcars that permit a relatively larger load to be carried on fewer railcar trucks. The cross section of the car is generally defined by the pair of spaced apart left and right hand deep side beams, and structure between the side sills of the side beams to support such lading as may be placed in the well. Typically the floor, or lading support structure in the well includes diagonally oriented members to carry shear between the side sills under lateral loading conditions.
Contemporary well cars may carry a number of alternative loads made up of containers in International Standards Association (ISO) sizes or domestic sizes, and of highway trailers. The ISO containers are 8′-0″ wide, 8′-6″ high, and come in a 20′-0″ length weighing up to 52,900 lbs., or a 40′-0″ length weighing up to 67,200 lbs. Domestic containers are 8′-6″ wide and 9′-6″ high. Their standard lengths are 45′, 48′ and 53′. All domestic containers have a maximum weight of 67,200 lbs. Recently 28′ long domestic containers have been introduced in North America. They are generally used for courier services which have lower lading densities. The 28′ containers have a maximum weight of 35,000 lbs.
Common sizes of highway trailers are the 28′ pup trailer weighing up to 40,000 lbs., and the 45′ to 53′ trailer weighing up to 60,000 lbs. for a two axle trailer and up to 90,000 lbs., for a three axle trailer. Hitches are located on the end structures at both ends of the well. The wheels of a trailer can rest in the well, with the front, or nose of the trailer overhanging the car end structure at one end or the other of the well. Where pup trailers are used, two back-to-back 28′ pup trailers can be loaded in the well facing in opposite directions. Alternatively, shipping containers, typically of 20 ft., 28 ft, or 40 ft lengths may be placed in the well, with other shipping containers stacked on top in a “double-stack” configuration. Further, well cars can carry mixed loads of containers and trailers.
Whichever the case may be, a well car is required to withstand three kinds of loads. First, it must withstand longitudinal draft and buff loads inherent in pulling or pushing a train, particularly those loads that occur during slack run-ins and run-outs on downgrades and upgrades. Other variations of the longitudinal load are the 1,000,000 lbs., squeeze load and the 1,250,000 lbs., single ended impact load. Second, the well car must support a vertical load due to the trailers or shipping containers it carries. Third, it must be able to withstand lateral loading as the well car travels along curves and switch turn-offs.
For example, in an earlier well car, as shown in U.S. Pat. No. 4,893,567 of Hill et al., issued Jan. 16, 1990, the structure between the side sills includes lateral cross members. The ends of the cross members are mounted to longitudinally extending side sills. The cross members are indirectly attached to the side sills via hinged fittings which, in turn, are attached to the side sills. The hinge connection may tend to permit some flexing of the structure under some loads, while still providing a connection conceptually analogous to a pin joint for resistance to lateral deflection.
Longitudinal compressive loads imposed on the well car are transmitted into the car at the draft gear stops in the coupler pocket; carried outboard in the end structures through the end shear plate, sills and bolsters to the side beams; and then along the top and bottom chords to the other end of the car. The combined compressive longitudinal loads alone, or in combination with the effect of the vertical container loads tend to urge the top chords to buckle. Typically under compressive loading the top chords of the side beams tend to move laterally inboard relative to the bottom chords.
One way to address this tendency is to employ top chords of heavier section and high polar moment of inertia. This may tend to increase the weight of the side beams. It is generally desirable to avoid increasing the weight of rail road cars, since an increase in weight implies an increase in cost of material for fabrication, increased running costs when the car is empty, and a reduced maximum lading capacity since the loaded weight of the car plus lading must not exceed a given limit, whether 263,000 lbs., 286,000 lbs., or 315,000 lbs., as may govern the service for which the car is intended. For these reasons, it is generally preferable to use a lesser weight of metal more efficiently.
The inward deflection of the top chords of the side beams under buckling loads (as suggested by the intermittently dashed lines exaggeratedly representing deflection, the top chord deflection being signified by ‘δ’ in FIG. 4a), can be resisted to some extent by providing an opposing spring mechanism. To that end, it is desirable to employ a continuous cross member from side to side, and side posts connecting the top and bottom chords. The attachment to the side sills is conceptually similar to that of a built-in end condition. That is, a built-in end condition occurs where the connection joint will not only carry a shear load, but will, in addition, transmit a bending moment. If the cross member transmits moments at connections to both side sills, and assuming that the cross member is of significant section relative to the side sills, then twisting of the side beams will tend to impose a bending load in the cross member. As the car is symmetrical, this moment may tend to be resisted by an equal and opposite moment arising in the other half of the car, as suggested by moment ‘M’, in FIG. 4a. When this occurs the cross member, and the other members in the load path, such as the side posts, co-operate to act as a spring assembly tending to resist the top chord deflection (buckling), and side beam twisting.
The floor structure of a container carrying well car may typically include lading bearing cross members (a) at the ends of the well in the 40 foot container pedestal positions, and (b) in the middle of the well in the form of a central cross member to support containers at the 20 foot position. These vertical load bearing cross members support the shipping container corners. The floor structure may also include several intermediate cross members, and diagonals. The intermediate cross members and diagonal members are conceptually like the members of a pin-jointed truss and are provided to aid in resistance to lateral loads, as opposed to bearing the vertical load of the containers. Consequently, inasmuch as these additional cross members perform a different function, they tend to be of significantly reduced section relative to the container bearing cross members.
In at least one earlier car, the connection of the floor cross members and diagonal members to the side sills has been the source of fatigue cracking concerns. When the cross members are welded in place, it is not uncommon for portions of the weld to be placed in repeated, cyclic loading during operation. Inasmuch as it is sometimes difficult to obtain consistent, defect-free welds, defects in the welds can provide fatigue crack initiation sites.
Use of hinges may tend to reduce the probability of fatigue crack initiation due to cyclic flexing in bending, since hinges do not transmit a bending moment. However, a hinged cross member may also not tend to function to resist the lateral flexing of the side sills particularly well. A bolted connection may be preferable to a welded connection, since it avoids the possibility of weld defects and high level of stress concentration due to geometric nonlinearities.
Other cross member assemblies, for example, as shown in U.S. Pat. No. 5,465,670 of Butcher, issued Nov. 14, 1995, similarly have connections to the side sills in the horizontal plane only. U.S. Pat. No. 5,465,670 shows a three part main cross member assembly having a linear section matingly engaged with a mounting bracket at either end. The mounting bracket is welded to the linear section and then attached to a horizontal leg of a side sill. Both the main cross members and corresponding single piece intermediate cross members have hollow rectangular cross-sections. No additional reinforcement is provided at the ends of either cross member where shear forces caused by lading are greatest.
The use of a three-part cross member at either the central, 20 foot container position at mid-span in the well between the rail car trucks, or at the 40 foot container pedestal positions as shown by Butcher, may also have disadvantages. Container support castings were connected to either end of an intermediate cross member at a pair of peripheral welds respectively. These welded joints were labour intensive and required full ultrasonic (UT) inspection. In service, the welds are subjected to relatively severe cyclic loading. Flaws in such welded joints may tend to become fatigue crack initiation sites when subjected to cyclic loading. It would be advantageous to employ a cross member at a container support position, whether at the 20 or 40 foot location, that tends not to expose a welded joint to cyclic loading. It would be most preferable to employ a forged (that is, hot or cold formed), one-piece monolithic beam that under-hangs the well from side sill to side sill.